1. Field of the Invention
The present invention relates to a network system, and more particularly to a filter apparatus of a domain name system (DNS) for managing a correspondence between an IP address and a domain name.
2. Related Background Art
A DNS (domain name system) is a service on a TCP/IP protocol for providing a host connected to a TCP/IP network with information of a name and an IP address, associated with each other, of the host connected in a network with a protocol (including a UDP (user datagram protocol) as a transport layer) using a TCP/IP (transport control protocol/Internet protocol) such as the Internet. In the DNS, a name called a domain name is used to put together hosts into an organization which they belong to; the domain name is hierarchichally appended to each organization type, each organization name, or each post in an organization such as a nation, a company, or a scientific or academic organization and the host name is assured of its uniqueness in the TCP/IP network by being combined with the domain name. For example, the WWW (world wide web) server of NEC Corporation which is a Japanese company connected to the Internet can be represented in a description form, “www.nec.co.jp” composed of “jp” indicating Japan, “co” indicating a company, “nec” indicating NEC Corporation, and “www” indicating a host name of the WWW server named in the company.
“nec.co.jp” of the “www.nec.co.jp” is a domain name indicating NEC Corporation allocated by an NIC (network information center) that is a domain name allocating institution in the Internet and “www” is a host name allocated in NEC Corporation. A host attempting to communicate using the TCP/IP protocol must know an IP address of a destination host and a host connected to the Internet attempting to connect to the WWW server using the TCP/IP protocol inquires of the DNS server an IP address corresponding to the name “www.nec.co.jp.” The host attempting to connect to “www.nec.co.jp” inquires first of the DNS server for managing information at the top of a domain hierarchical structure in a DNS called a root server so as to be informed of a DNS server for managing the “jp” domain, next inquires of the DNS server for managing the “jp” domain to be informed of a DNS server for managing the “co.jp” domain, subsequently inquires of the DNS server for managing the “co.jp” domain to be informed of a DNS server for managing the “nec.co.jp” domain, and inquires an IP address corresponding to the host name “www.nec.co.jp” of the DNS server for managing the “nec.co.jp” domain and then the IP address of the host is returned if the name exists in the DNS server.
In an organization connected to the Internet, a firewall may be installed in some cases in a portion connected to the Internet to restrict communications directly to the outside of the organization with the TCP/I protocol for a security reason.
As a security requirement of an organization, there is a restriction against accesses from the outside of the organization to resources inside the organization with the TCP/IP protocol for a protection of information secret against the outside of the organization.
The DNS is also required to hide a name of the host connected to the network inside the organization, information on an IP address, and a domain name indicating a post name or a network configuration of the organization as completely as possible so as to prevent a network invader from invading the network inside the organization by using the information.
A conventional system meets the above requirement by installing a DNS server for providing information on a host for authorizing an access from a host outside the organization installed in the outside of the firewall in addition to the DNS server inside the organization, making settings for the DNS server inside the organization so that the host inside the organization can inquire recursively of the DNS server installed outside the firewall to obtain DNS information of the host outside the organization, and making settings for the DNS server and the firewall so that the DNS server installed outside the firewall cannot make an inquiry to the DNS server inside the organization.
The conventional system having this constitution causes problems of a need for installing a plurality of DNS servers and complicated management of the DNS servers.
As a security problem, protective measures are required against an attack called DoS (denial of service) attack which stops a service due to a problem on an implementation of a server program such as bugs by transmitting a packet in an illegal format to a attack-targeted server, and a necessity of these measures is pointed out for the DNS service.
Conventionally, if this kind of problem is pointed out, a developer of a service program must modify the service program.
Certainly, source files of a part of service programs are open to the public (bind for UNIX TM, etc.) and therefore it is said that the service programs can be replaced with ones coping with the DoS attack by a user who obtains a modified part different from the source or by a user who modifies the part and compiles the service programs.
If the source file is not open to public (for example, a DNS server included in Windows NT Server 4.0 made by Microsoft Corporation), however, a long time is taken until a service program developer distributes modification modules to service program users and it has been a long time since the DoS attack or other problems are pointed out without appropriately coping with these problems.
Furthermore, even if a source file is open to public, a user cannot cope with these problems due to his or her insufficient programming skill or the like in some cases.
While the DoS attack has been described hereinabove, the same problems may occur unless a normal response to be obtained intrinsically is received due to a problem on an implementation of the service program even though the service is not stopped.
In addition, from a viewpoint of a network security management of organizations, there are some organizations imposing a security requirement of countermeasures for inhibiting anyone inside the organization from making an attack which can be a menace on a security against a host outside the organization.
In U.S. Pat. No. 5,805,820, there are provided in a DNS a method of inhibiting a transmission of private information such as a domain name and an IP address of a network inside an organization to the outside of the organization through a DNS by redirecting an inquiry request to the inside information of the domain and an apparatus for realizing it. It is, however, not capable of coping with the problems of the DoS attack or the like.